Blame the Gerbils? Blame the Journalists!*

It would appear that a lot of people are actually interested in the accurate reporting of research results. YOU HEAR THAT ‘THE MEDIA’?

Therefore, I want to take this opportunity to discuss how in the heck the Schmid et al study stories went oh so very wrong (as demonstrated in my previous post) – leaving aside again any discussion on the actual results of the study (although you can get a sneak peak in the comments of that previous post).

The first few news stories actually reported this study pretty well. But while some went down the ‘plague driven by climate’ and ‘plague reintroduced multiple times’ route with their headlines, others (even though they report the results similarly to the stories above) used phrases that could be read as ‘gerbils to blame for plague’ and ‘gerbils replace rats as cause of plague’.

Most* articles written after this point read like a game of telephone/Chinese Whispers; journalists taking the story from each other without fact-checking, either with the press release or the article itself. Each one makes a little change here or there and eventually the stories are claiming things the research/ers never did, most notably ‘gerbils brought the plague to Europe’.

This is a pretty common occurrence in journalism. That’s not to say it’s excusable (I mean, I’m not a journalist and I managed to get it right), but it happens. However, I do think there are other issues that need to be considered.

If at this point you need/want to read a summary of the study, you can find a full one in my previous post on this subject (1500 words) or a short and snappy one here (550 words).

The Press Release

Similar to above, this press release features a practice that is common in public relations and one that I suspect is the root of many mis-represented study results: definitive language. While the paper itself does a reasonable job of stating their results within the context of the study (for example, their conclusions are based on one tree-ring sample), the press release makes it sound like this absolutely the way it happened, no questions asked. On the great gerbil front, they are mentioned at the beginning of one paragraph as a species of rodent known in Asia to start plague outbreaks. This paragraph then ends by suggesting that the disease spread to Europe in the medieval period multiple times when “plague outbreaks in one of Asia’s natural rodent reservoirs reached the trade routes, and spilled over into human civilization.” It doesn’t exactly make it clear they actually meant: the rodents in Asia with plague (which may or may not have been great gerbils) die [HERE ENDS THE INVOLVEMENT OF ANY IMAGINARY GERBILS], the fleas that live on them find new hosts, [AFTER THIS WE SPECULATE] these hosts might have been camels, the camels spread the plague over land via caravans, the caravans spread the plague to ships, the ships bring the plague to Europe over sea (possibly because of rats, but definitely not because of gerbils).

[Note, the actual paper abstract isn’t super clear on the above either, but then again it doesn’t say ‘gerbils’ anywhere, so that’s at least something – and in their own news article on the story, the authors of the study actually say, “we don’t yet know exactly how the plague made the journey westward”!]

Agent, Host, Vector, Reservoir

I also think the ‘gerbils brought the plague to Europe’ issue may stem, in part, from many people misunderstanding the various epidemiological terms that are used throughout articles on this study. Even though gerbils are not the only species mentioned in the paper with regards to rodent reservoirs (ground squirrels and Altai marmots are as well), understanding the definition of reservoir is really important for this study. So here’s a quick run-down of some terms:

Agent

An agent is the cause of a disease. In infectious diseases the agent is usually a microbe – a really small organism – which could be bacteria, virus, fungus, prions, or a type of parasite called protozoa. These are called infectious agents. The plague is caused by bacteria called Yersinia pestis.

Host

Hosts are also organisms, usually humans or other animals, which are infected with an agent. The agent may make the host sick – or the host may not get sick and act as a carrier for an agent. Many different organisms are known to be hosts for the plague bacteria. Three of the most well known are probably humans, rats, and fleas. Lots of rodent species can be hosts, such as: squirrels, marmots, prairie dogs, chipmunks, mice, voles, and yes… gerbils. And other larger species that can be hosts include (but are not limited to): rabbits, cats, dogs, sheep, goats, and yes… camels.

Vector

Any living organism that carries and then transmits an infectious agent to another living organism is a vector. In this sense, all vectors are hosts (but not all hosts are vectors, as they may not all transmit the agent). A lot of research has suggested that fleas are one of the main vectors of the plague bacteria. Vectors will often transmit an agent from a host inside the reservoir to a host that is outside of the reservoir.

Reservoir

A reservoir is any organism or substance, such as humans, animals, plants, soil or a combination of these, in which an infectious agent normally lives – and mostly depends on for its survival, because the agent is able to multiply there. In the case of the plague bacteria, many species of rodents are reservoirs. This is because they are often susceptible to infection (can become hosts for the agent) but are resistant to the disease (do not get sick, so they become carriers). Areas that have persistent reservoirs in them are called foci. There are plague foci on every continent except Australia.

Natural plague foci (in rodent populations) – from WHO: http://www.who.int/csr/resources/publications/plague/CSR_ISR_2000_1/en/

I feel like maybe some people (including journalists) are confusing reservoirs for vectors and hosts! Schmid et al do not think there is any evidence that rats were a reservoir for the plague bacteria in Europe during the medieval period. Instead, they think the reservoirs were in Asia (in an unspecified rodent species population) and that the plague bacteria was spread into Europe from these reservoirs through different vectors and hosts. To be very clear, this confusion is totally understandable – but if you’re writing about this story, you should probably know to check.

Snappy Summary**

You may have read in the headlines or heard from someone that researchers have said ‘gerbils brought the plague to Europe’ and that ‘rats were unfairly blamed’. Unfortunately, this isn’t exactly what their study showed (or what they said).

Rather, the researchers suggest that the plague bacteria may have been brought to Europe, starting with the Black Death in 1347 and then again many different times until the disease stopped happening in Europe around the mid-1800s.

So how do gerbils come into it? Well, the researchers decided to look for evidence that rats in Europe were reservoirs for the plague during this time. This would mean that the plague bacteria could be introduced once in 1347 and then rats would keep it going for many hundreds years so that it could be spread to humans over and over again, usually by fleas.

Modern research shows another plague reservoir species, great gerbils (Rhombomys opimus) react to changes in climate in a way that might cause outbreaks of the disease in humans. Warm and wet weather results in more plants around for great gerbils to eat, so their populations boom – and so do the numbers of fleas that live on them. If the climate changes to something less good for the gerbils, many of them may die – causing the fleas that live on them to find a new host (which increases the chances for humans to become infected).

Because of this the researchers decided to look for similar climate events (warm and wet, followed by a change) in the medieval period in Europe. They did this by looking at nine tree ring samples, but they didn’t find any climate events that matched what they were looking for.

So instead they decided to look for these climate events outside of Europe, in Asia. This is because plague reservoirs still exist in many different rodent species in Asia and it is thought they were there in the medieval period too. They looked at six tree ring samples and one of them, from the mountainous regions of Pakistan, had some of these climate events (28 in total).

When they compared the dates of these climate events from the tree rings to outbreaks of the plague in Europe, they found a pattern. Approximately 15 years after nine of those climate events happened there were 11 outbreaks of plague in different European harbours between 1346 – 1837.

Therefore, the researchers think the following might have happened. The climate events that showed up in the tree rings from Pakistan may have impacted a bigger area of Asia. The reservoir populations in this area would experience the boom and then bust described above, causing the fleas to find new hosts – outside of the reservoir. THIS IS WHERE THE ROLE OF THE RESERVOIR (WHICH IS POTENTIALLY, BUT NOT DEFINITELY GREAT GERBILS) ENDS.

The researchers don’t know for sure how the plague bacteria then spread into Europe, but they think some of these fleas may have infected camels in caravans. The caravans travelled the trading routes across Asia, spreading the disease to each other through their camels, other animals, people, and fleas, before it finally ended up in harbours. From here, some infected things (they are less clear about what, but maybe people, rats, and fleas) would get on ships and sail to Europe, spreading the disease.

Now the researchers want to look at plague DNA from skeletons of plague victims to try and prove they’re right.

—

*Not all of them. Some did a really good job and for others, well… it’s not entirely their fault.

** Right, so yesterday I wrote a full summary of the Schmid et al study, which I am pleased to see that many people are finding interesting and useful. However, I am aware that if someone asks you to explain ‘why the headlines are wrong’ reading them a 1500 word piece isn’t exactly helpful. Therefore, I have tried to write another (shorter) summary that is just tries to get the gist across (but without being inaccurate).